When designing microwave reaction cavities it is assumed that the filling dielectric is air (where .epsilon.=1) and the dimensions and shape of the cavity are normally selected on this basis. It is, however, known that the effect of a dielectric material must be considered in the design of a waveguide and similar components whose dimensions must be related to the wavelength of the electromagnetic energy within the component. The presence of a dielectric where permittivity is greater than that of air (.epsilon.&gt;1), for example, will cause the wavelength within the dielectric to be shorter than that in air. In this way, waveguides or chambers of fixed physical dimensions will appear to be effectively larger when filled with dielectric material of higher permittivity, because of an apparent increase in effective size due to the increased number of wavelengths accommodated in the dielectric. The scaling effect is proportional to the square root of the relative permittivity (.epsilon..sub.r) of the dielectric.
It has been known for some time that certain metallurgical effects can be brought about in metal bearing ores and mineral concentrates by treatment with microwaves such that the ore or concentrate becomes more amenable to conventional recovery techniques, such as leaching. For example, it is known that refractory gold concentrates can be treated with microwaves to transform pyrites into pyrrhotite and hematite, the latter being more reactive than the former and thus more readily processed by conventional techniques. Similar processes have been carried out at bench scale for the recovery of molybdenum and rhenium from their sulphide ores; recovery of nickel, cobalt and manganese from their oxides and silicates; and recovery of copper from its ores.
Fluidized bed reactors are presently widely used in many ore processing applications where strong interaction between a solid product and gas medium is required and the use of microwave energy to provide some or all of the required reaction energy has been disclosed in, for example, U.S. Pat. Nos. 3,528,179, 4,126,945, 4,967,486, 4,511,362, 4,476,098, 5,382,412 and 5,051,456.
Where microwave energy is used in association with a fluidized bed reactor, the reaction chamber, or cavity, typically consists of at least two zones; one is the region near the bed or base of the reactor where the fluidizing gas is usually introduced into the material and includes the mass of the material in its suspended state (the reaction zone). The second zone is the region above the reacting mass of material, consisting primarily of the fluidizing gas or gaseous reaction products but containing comparatively little of the mass of material. The delineation of these two zones may be established by adjusting the fluidizing pressure or gas velocity so as to cause the reacting material not to occupy the upper vessel region; alternatively, this delineation may be established through use of a filter screen assembly which prevents particle flow into the upper vessel region but which is otherwise transparent to microwave energy.
The delineation of these two zones within the reaction chamber is especially pronounced when the load of material or its charge consists of a relatively dense dielectric of high permittivity such as granulated ores, soils, etc. The coexistence of two or more regions of differing dielectric properties within the chamber will generally result in a situation where the electromagnetic energy cannot transmit smoothly from one region to another, rather what usually occurs is a significant reflection of wave energy at the boundaries between the regions and a complete redistribution of energy throughout the regions. This will result in less than optimal energy transfer into the material by the microwaves.